Flat panel displays are of immense importance in electronics. In current developments, Active Matrix Liquid Crystal Displays (AMLCD) are beginning to challenge the dominance of Cathode Ray Tube (CRT) technology. AMLCD devices are non-emissive and require complex lithography. Filters and matching spectral backlights are required to produce colour. However, there are many light losses and inherent complexity in AMLCD devices because of the non-linear nature of liquid crystal materials. This results in a display that is less bright than CRT with a smaller colour gamut and poorer viewing angle and contrast. Also, due to the non-emissive nature of the display, inefficient use of input electrical power is made often with well over 70% of the energy being lost as non-useful energy.
Field emission displays, based on conventional ‘Spindt Tip’ technology, promised a solution to flat panel display problems. Field emission displays (FEDs) are essentially flat cathode ray tube (CRT) devices. However, rather than one electron gun firing electrons at a phosphor on a screen through a shadow mask, the FED has tens or hundreds of individual tip in each display pixel. The tips are known as Spindt tips, after the inventor Cap Spindt. The process of fabrication relies on defining a pattern of holes in a gate metal by photolithography. An underlying insulator is then etched in an isotropic wet etch that “undercuts” leaving a well beneath the metal. A sacrificial layer (usually nickel) is then evaporated on the surface at an oblique angle to ensure the well is not filled. The emitter material (usually tungsten or molybdenum) is then evaporated through the holes in to the well. As the evaporate metal builds up on the surface, on the sacrificial layer, it closes the hole as the thickness increases, and has the effect of providing an emitter tip in the well. The top metal is then removed by etching the sacrificial layer, leaving the tip, the well, and the original gate metal. This forms the backplate of Spindt tips. A top plate containing a patterned phosphor is then placed relative to the backplate using spacers. The final device is evacuated to allow the emitted electrons a long mean free path. The principle of field emission from micro-tips is well understood and is governed by Fowler-Nordheim tunneling. The emission current, and therefore brightness of the display, depends then only on the current density, the number of tips and their sharpness, i.e.I=JFNnαWhere n=number of tips, α the tip sharpness and JFN the Fowler-Nordheim tunnel current density.
The tips will provide a sharp electron source that will provide hot electron injection into, for example, a phosphor.
Unfortunately, the extreme complication in fabrication has limited the use of this technology. Additionally, crystal silicon emitters are limited by the wafer size.
Other thin-film materials may also be used for field emission. Carbon is the main contender with diamond, diamond like carbon and carbon nano-tubes also suitable. The use of diamond seemed a good choice, although this is difficult to fabricate and also the mechanism of a supposed negative electron affinity which diamond was claimed to have has now been questioned.
An object of at least one embodiment of at least one aspect of the present invention is to obviate or at least mitigate at least one of the aforementioned problems in the prior art.